+ mm-page-writeback-avoid-overflow-when-reporting-bdi-byte-limits.patch added to mm-new branch

+ mm-page-writeback-avoid-overflow-when-reporting-bdi-byte-limits.patch added to mm-new branch

[Andrew Morton]

The patch titled
     Subject: mm/page-writeback: avoid overflow when reporting BDI byte limits
has been added to the -mm mm-new branch.  Its filename is
     mm-page-writeback-avoid-overflow-when-reporting-bdi-byte-limits.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-page-writeback-avoid-overflow-when-reporting-bdi-byte-limits.patch

This patch will later appear in the mm-new branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Note, mm-new is a provisional staging ground for work-in-progress
patches, and acceptance into mm-new is a notification for others take
notice and to finish up reviews.  Please do not hesitate to respond to
review feedback and post updated versions to replace or incrementally
fixup patches in mm-new.

The mm-new branch of mm.git is not included in linux-next

If a few days of testing in mm-new is successful, the patch will me moved
into mm.git's mm-unstable branch, which is included in linux-next

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------------------------------------------------------
From: Hungyu Lin <dennylin0707@gmail.com>
Subject: mm/page-writeback: avoid overflow when reporting BDI byte limits
Date: Tue,  3 Nov 2026 19:36:26 +0000

bdi_get_bytes() multiplies unsigned long operands before assigning the
result to u64.  On 32-bit kernels, the intermediate product wraps and
min_bytes and max_bytes report incorrect limits.  For example, with
vm.dirty_bytes set to 104857600 and max_ratio set to 100, max_bytes
reports 268 instead of 104857600.

Convert the page count to bytes in u64 and use mul_u64_u32_div() to avoid
overflow in the subsequent multiplication.  This also handles large dirty
limits on systems with larger pages.  Combining the two divisions keeps
the existing rounding behavior.

Link: https://lore.kernel.org/20261103193627.71959-1-dennylin0707@gmail.com
Fixes: 00df7d51263b ("mm: add bdi_get_max_bytes() function")
Signed-off-by: Hungyu Lin <dennylin0707@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Stefan Roesch <shr@devkernel.io>
Cc: <stable@vger.kernel.org>
---

 mm/page-writeback.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/mm/page-writeback.c~mm-page-writeback-avoid-overflow-when-reporting-bdi-byte-limits
+++ a/mm/page-writeback.c
@@ -665,7 +665,8 @@ static u64 bdi_get_bytes(unsigned int ra
 	u64 bytes;
 
 	global_dirty_limits(&background_thresh, &dirty_thresh);
-	bytes = (dirty_thresh * PAGE_SIZE * ratio) / BDI_RATIO_SCALE / 100;
+	bytes = mul_u64_u32_div((u64)dirty_thresh << PAGE_SHIFT, ratio,
+				100 * BDI_RATIO_SCALE);
 
 	return bytes;
 }
_

Patches currently in -mm which might be from dennylin0707@gmail.com are

mm-page-writeback-avoid-overflow-when-reporting-bdi-byte-limits.patch
delayacct-preserve-delay-extrema-when-aggregating-taskstats.patch
shmem-clamp-negative-used-block-samples-in-statfs.patch